Piezoelectric bulk wave device, and method of manufacturing the piezoelectric bulk wave device

ABSTRACT

A piezoelectric bulk wave device that includes a piezoelectric thin plate that is made of LiTaO 3 , and first and second electrodes that are provided in contact with the piezoelectric thin plate. The piezoelectric bulk wave device utilizes the thickness shear mode of the piezoelectric thin plate made of LiTaO 3 . The first and second electrodes are each formed by a conductor having a specific acoustic impedance higher than the specific acoustic impedance of a transversal wave that propagates in LiTaO 3 . When the sum of the film thicknesses of the first and second electrodes is defined as an electrode thickness, and the thickness of the piezoelectric thin plate made of LiTaO 3  is defined as an LT thickness, the electrode thickness/(electrode thickness+LT thickness) is not less than 5% and not more than 40%.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of International applicationNo. PCT/JP2012/071576, filed Aug. 27, 2012, which claims priority toJapanese Patent Application No. 2011-190342, filed Sep. 1, 2011, theentire contents of each of which are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a piezoelectric bulk wave device usingLiTaO₃, and a method of manufacturing the piezoelectric bulk wavedevice. More specifically, the present invention relates to apiezoelectric bulk wave device that utilizes a bulk wave of thicknessshear mode as a bulk wave, and a method of manufacturing thepiezoelectric bulk wave device.

BACKGROUND OF THE INVENTION

In related art, piezoelectric thin film devices are used foroscillators, filters, and the like. For example, a piezoelectric thinfilm device illustrated in FIG. 24 is disclosed in Patent Document 1mentioned below. A piezoelectric thin film device 1001 has apiezoelectric thin film 1002. It is described in Patent Document 1 thatthe piezoelectric thin film 1002 is desirably made of piezoelectricmonocrystal, such as quartz crystal, LiTaO₃, or LiNbO₃. Electrodes 1003and 1004 are formed on the upper surface of the piezoelectric thin film1002. Electrodes 1005 to 1007 are formed on the lower surface of thepiezoelectric thin film 1002. By using the electrodes 1003 to 1007, fourpiezoelectric thin film resonators that utilize thickness shear mode areformed in the piezoelectric thin film device 1001.

-   Patent Document 1: Japanese Unexamined Patent Application    Publication No. 2007-228356

SUMMARY OF THE INVENTION

As described in Patent Document 1, in related art, piezoelectric thinfilm devices using the thickness shear mode of LiTaO₃ are known.However, in a case where the thickness shear mode is utilized by using apiezoelectric thin film made of LiTaO₃, it is difficult to obtain goodelectrical characteristics. Moreover, manufacturing variations in LiTaO₃or electrode film thickness tend to lead to large variations inelectrical characteristics.

It is an object of the present invention to provide a piezoelectric bulkwave device that utilizes the thickness shear mode of LiTaO₃, andexhibits relatively small variations in electrical characteristicscaused by variations in electrode film thickness or in the thickness ofLiTaO₃, and a method of manufacturing the piezoelectric bulk wavedevice.

A piezoelectric bulk wave device according to the present inventionutilizes the thickness shear mode of a piezoelectric thin plate made ofLiTaO₃. The piezoelectric bulk wave device according to the presentinvention has a piezoelectric thin plate that is made of LiTaO₃, andfirst and second electrodes that are provided in contact with thepiezoelectric thin plate. According to the present invention, the firstand second electrodes are each formed by a conductor having a specificacoustic impedance higher than a specific acoustic impedance of atransversal wave that propagates in LiTaO₃, and when a sum of filmthicknesses of the first and second electrodes is defined as anelectrode thickness, and a thickness of the piezoelectric thin platemade of LiTaO₃ is defined as an LT thickness, the electrodethickness/(electrode thickness+LT thickness) is in a range of not lessthan 5% and not more than 40%.

In a specific aspect of the piezoelectric bulk wave device according tothe present invention, each of the first and second electrodes is atleast one metal selected from the group consisting of W, Mo, Pt, and Taor an alloy mainly including the metal, or a laminate including themetal that accounts for more than half of the laminate in weight ratio.

A method of manufacturing a piezoelectric bulk wave device according tothe present invention is a method of manufacturing a piezoelectric bulkwave device that utilizes a thickness shear mode of a piezoelectric thinplate made of LiTaO₃, and includes the steps of preparing apiezoelectric thin plate, forming a first electrode in contact with thepiezoelectric thin plate, the first electrode being formed by aconductor having a specific acoustic impedance higher than a specificacoustic impedance of a transversal wave that propagates in LiTaO₃, thefirst electrode being configured so that an electrodethickness/(electrode thickness+LT thickness) is not less than 5% and notmore than 40%, and forming a second electrode in contact with thepiezoelectric thin plate, the second electrode being formed by aconductor having a specific acoustic impedance higher than the specificacoustic impedance of the transversal wave that propagates in LiTaO₃,the second electrode being configured so that an electrodethickness/(electrode thickness+LT thickness) is not less than 5% and notmore than 40%.

In a specific aspect of the method of manufacturing a piezoelectric bulkwave device according to the present invention, the step of preparingthe piezoelectric thin plate includes the steps of implanting ions fromone side of a piezoelectric substrate made of LiTaO₃ to form a highconcentration ion-implanted portion on the one side, the highconcentration ion-implanted portion being a portion of highestimplanted-ion concentration, joining a support substrate to the one sideof the piezoelectric substrate, and separating the piezoelectricsubstrate at the high concentration ion-implanted portion while heatingthe piezoelectric substrate, into a piezoelectric thin plate thatextends from the one side of the piezoelectric substrate to the highconcentration ion-implanted portion, and a remaining piezoelectricsubstrate portion.

In this case, the position of the high concentration ion-implantedportion can be easily controlled by selecting the above-mentioned ionimplantation conditions. Generally, an ion beam from an ion implantationdevice is linearly applied to the substrate with uniform intensity, andthe ion beam is applied a number of times at substantially the sameirradiation angle to the same location while scanning the entiresubstrate. Accordingly, in comparison to thickness variations due tofilm deposition methods such as sputtering, CVD, and evaporation, byusing ion implantation, the position of the high concentrationion-implanted portion becomes uniform over the entire substrate surface,leading to smaller variations in depth. Therefore, it is possible toeasily obtain a piezoelectric thin plate with an accurate thickness overthe entire substrate surface.

In another specific aspect of the method of manufacturing apiezoelectric bulk wave device according to the present invention, thestep of preparing the piezoelectric thin plate includes the steps ofimplanting ions from one side of a piezoelectric substrate made ofLiTaO₃ to form a high concentration ion-implanted portion on the oneside, the high concentration ion-implanted portion being a portion ofhighest implanted-ion concentration, bonding a temporary support memberonto the one side of the piezoelectric substrate, and separating thepiezoelectric substrate at the high concentration ion-implanted portionwhile heating the piezoelectric substrate bonded on the temporarysupport member, into a piezoelectric thin plate that extends from theone side of the piezoelectric substrate to the high concentrationion-implanted portion, and a remaining piezoelectric substrate portion,and the method includes the step of detaching the temporary supportmember from the piezoelectric thin plate.

In this case, it is possible to reduce the possibility of a defectoccurring in the piezoelectric thin plate due to thermal stress exertedwhen separating the piezoelectric thin plate.

In still another specific aspect of the method of manufacturing apiezoelectric bulk wave device according to the present invention, priorto detaching the temporary support member from the piezoelectric thinplate, the steps of forming a first electrode on the piezoelectric thinplate, forming a dummy layer so as to cover the first electrode, andlaminating a support substrate on the dummy layer are performed. Inaddition, the method further includes the steps of, after detaching thetemporary support member from the piezoelectric thin plate, forming asecond electrode on another side of the piezoelectric thin plate whichis exposed by the detaching of the temporary support member, and causingthe dummy layer to disappear. In this case, in accordance with thepresent invention, it is possible to provide a piezoelectric bulk wavedevice having a structure in which a vibrating part having the first andsecond electrodes formed on the top and the bottom of the piezoelectricthin plate is made afloat from the support substrate.

According to the present invention, the first and second electrodes areeach formed by a conductor having a specific acoustic impedance higherthan the specific acoustic impedance of a transversal wave thatpropagates in LiTaO₃, and when the sum of the film thicknesses of thefirst and second electrodes is defined as an electrode thickness, andthe thickness of the piezoelectric thin plate made of LiTaO₃ is definedas an LT thickness, the electrode thickness/(electrode thickness+LTthickness) is in the range of not less than 5% and not more than 40%.Therefore, it is possible to provide a piezoelectric bulk wave devicethat exhibits relatively small fluctuations in electromechanicalcoupling coefficient. Accordingly, it is possible to provide apiezoelectric bulk wave device with relatively small variations incharacteristics.

In the method of manufacturing a piezoelectric bulk wave deviceaccording to the present invention, the first and second electrodes areconfigured in the manner as mentioned above. Therefore, in accordancewith the present invention, it is possible to provide a piezoelectricbulk wave device with relatively small variations in characteristics.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1( a) and 1(b) are an elevational cross-sectional view and a planview, respectively, of a piezoelectric bulk wave device according to anembodiment of the present invention.

FIG. 2 illustrates the relationship between electrodethickness/(electrode thickness and LT thickness), the electrodethickness being the electrode thickness of first and second electrodesmade of W, and resonant frequency, in accordance with an embodiment ofthe present invention.

FIG. 3 illustrates the relationship between electrodethickness/(electrode thickness and LT thickness), the electrodethickness being the electrode thickness of the first and secondelectrodes made of W, and electromechanical coupling coefficient k², inaccordance with an embodiment of the present invention.

FIG. 4 illustrates the relationship between electrodethickness/(electrode thickness and LT thickness), the electrodethickness being the electrode thickness of the first and secondelectrodes made of W, and damping capacitance C₀, in accordance with anembodiment of the present invention.

FIG. 5 illustrates the relationship between upper electrodethickness=lower electrode thickness, and the thickness of LiTaO₃, in acase where the first and second electrodes are made of W and theresonant frequency is 880 MHz in accordance with an embodiment of thepresent invention.

FIG. 6 illustrates the relationship between upper electrodethickness=lower electrode thickness, and damping capacitance C₀, in acase where the first and second electrodes are made of W and theresonant frequency is 880 MHz in accordance with an embodiment of thepresent invention.

FIG. 7 illustrates the relationship between upper electrodethickness=lower electrode thickness, and electromechanical couplingcoefficient k², in a case where the first and second electrodes are madeof W and the resonant frequency is 880 MHz in accordance with anembodiment of the present invention.

FIG. 8 illustrates the relationship between upper electrodethickness=lower electrode thickness, and the thickness of LiTaO₃, in acase where the first and second electrodes are made of W and theresonant frequency is 1960 MHz in accordance with an embodiment of thepresent invention.

FIG. 9 illustrates the relationship between upper electrodethickness=lower electrode thickness, and damping capacitance C₀, in acase where the first and second electrodes are made of W and theresonant frequency is 1960 MHz in accordance with an embodiment of thepresent invention.

FIG. 10 illustrates the relationship between upper electrodethickness=lower electrode thickness, and electromechanical couplingcoefficient k², in a case where the first and second electrodes are madeof W and the resonant frequency is 1960 MHz in accordance with anembodiment of the present invention.

FIG. 11 illustrates the relationship between the Euler Angles θ ofLiTaO₃ and specific acoustic impedance.

FIG. 12 illustrates the relationship between the Euler Angles θ ofLiTaO₃, and the resonant frequency Fr and anti-resonant frequency Fa ofthickness shear mode.

FIG. 13 illustrates the relationship between the Euler Angles θ ofLiTaO₃, and the electromechanical coupling coefficient k² of thicknessshear mode.

FIG. 14 illustrates the relationship between the Euler Angles θ ofLiTaO₃, and the temperature coefficient of frequency TCF of thicknessshear mode.

FIG. 15 illustrates the relationship between the Euler Angles θ ofLiTaO₃, and the electromechanical coupling coefficient ksp² of thicknesslongitudinal vibration mode that is spurious.

FIG. 16 illustrates the relationship between the Euler Angles θ ofLiTaO₃, and the resonant frequency Fr and anti-resonant frequency Fa ofthickness longitudinal vibration.

FIG. 17 illustrates the relationship between the Euler Angles θ ofLiTaO₃, and the electromechanical coupling coefficient k² of thicknesslongitudinal vibration.

FIG. 18 illustrates the relationship between the Euler Angles θ ofLiTaO₃, and temperature coefficient of frequency TCF.

FIG. 19 illustrates the relationship between Euler Angles θ andtemperature coefficient of frequency TCF in thickness shear vibrationmode, in a case where LiNbO₃ is used.

FIGS. 20( a) to 20(d) are schematic elevational cross-sectional viewsfor explaining a method of manufacturing a piezoelectric bulk wavedevice according to an embodiment of the present invention.

FIGS. 21( a) to 21(c) are schematic elevational cross-sectional viewsfor explaining a method of manufacturing a piezoelectric bulk wavedevice according to an embodiment of the present invention.

FIGS. 22( a) to 22(c) are schematic elevational cross-sectional viewsfor explaining a method of manufacturing a piezoelectric bulk wavedevice according to an embodiment of the present invention.

FIGS. 23( a) to 23(c) are schematic elevational cross-sectional viewsfor explaining a method of manufacturing a piezoelectric bulk wavedevice according to an embodiment of the present invention.

FIG. 24 is a schematic cross-sectional view illustrating an example of apiezoelectric thin film device according to related art.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, the present invention will be explained by way of specificembodiments of the present invention with reference to the drawings.

FIGS. 1( a) and 1(b) are a schematic elevational cross-sectional viewand a plan view, respectively, of a piezoelectric bulk wave deviceaccording to an embodiment of the present invention.

A piezoelectric bulk wave device 1 according to this embodiment has asupport substrate 2. The support substrate 2 is formed by a suitableinsulating body or piezoelectric body. In this embodiment, the supportsubstrate 2 is formed by alumina.

An insulating layer 3 is formed on the support substrate 2. While theinsulating layer 3 is made of silicon oxide in this embodiment, theinsulating layer 3 may be made of a suitable insulating material such asLiTaO₃, LiNbO₃, sapphire, or glass. Alumina, glass, and LiNbO₃ arepreferred because these materials are inexpensive in comparison toLiTaO₃ and sapphire, and easy to manufacture. A recess 3 a is formed onthe upper surface of the insulating layer 3. A piezoelectric thin platevibrating part 4 is disposed above the portion where the recess 3 a isprovided. The piezoelectric thin plate vibrating part 4 has apiezoelectric thin plate 5, a first electrode 6 formed on the uppersurface of the piezoelectric thin plate 5, and a second electrode 7formed on the lower surface of the piezoelectric thin plate 5.

The piezoelectric thin plate 5 is made of LiTaO₃. The piezoelectric thinplate 5 refers to a thin piezoelectric body with a thickness of not morethan 2 μm. According to a manufacturing method described later, such apiezoelectric thin plate made of LiTaO₃ having a small thickness can beobtained with high accuracy by using an ion implantation-splittingmethod.

The piezoelectric thin plate 5 is a piezoelectric body having a smallthickness of not more than about 2 μm as mentioned above. According tothis embodiment, in a case where such as a piezoelectric thin plate madeof LiTaO₃ and having a small thickness is used, even if variations occurin the thickness of the LiTaO₃, variations in electromechanical couplingcoefficient k² can be reduced. This is because the first and secondelectrodes 6 and 7 are configured as described below according to thisembodiment.

That is, according to a characteristic feature of the present invention,there is provided a piezoelectric bulk wave device that utilizes a bulkwave of the thickness shear mode of LiTaO₃, in which the first andsecond electrodes 6 and 7 are each formed by a conductor with a specificacoustic impedance higher than the specific acoustic impedance of atransversal wave that propagates in LiTaO₃, and when the sum of thethicknesses of the first and second electrodes 6 and 7 is defined as anelectrode thickness, and the thickness of LiTaO₃ is defined as an LTthickness, the electrode thickness/(electrode thickness+LT thickness) isnot less than 5% and not more than 40%. In this case, variations inelectromechanical coupling efficient k² can be reduced. That is, becausevariations in electromechanical coupling efficient k² are small even inthe presence of variations in electrode 6 and 7 thickness or LiTaO₃thickness, fluctuations in fractional band width can be reduced.

Further, in a case where the electrode thickness/(electrode thickness+LTthickness) mentioned above is higher than or equal to 5%, theelectromechanical coupling coefficient k² can be increased to be higherthan or equal to 10%. These features will be described in more detailbelow with reference to FIGS. 2 to 10.

FIGS. 2 to 10 assume the following configuration.

A structure in which the first electrode 6 made of W is laminated on theupper surface of a piezoelectric thin plate made of LiTaO₃, and thesecond electrode 7 is laminated on the lower surface of thepiezoelectric thin plate. It is supposed that the thickness of LiTaO₃,that is, the LT thickness, and the electrode thickness=(the thickness ofthe first electrode 6 and the thickness of the second electrode 7) are1000 nm. That is, the electrode thickness=upper electrodethickness+lower electrode thickness. In this case, the electrodethickness/(electrode thickness+LT thickness) was varied within the rangeof 5% to 95%. It is supposed that the first electrode 6 and the secondelectrode 7 are opposed to a square-shaped region with an area of 44.7μm×44.7 μm=2000 μm², and also, the Euler Angles of LiTaO₃ were set as(0°, 73°, 0°).

FIG. 2 illustrates the relationship between the electrodethickness/(electrode thickness+LT thickness) mentioned above, andresonant frequency. FIG. 3 illustrates the relationship between theelectrode thickness/(electrode thickness+LT thickness) mentioned above,and electromechanical coupling coefficient k². FIG. 4 illustrates therelationship between the electrode thickness/(electrode thickness+LTthickness) mentioned above, and damping capacitance C₀.

It is apparent from FIG. 3 that when the electrode thickness/(electrodethickness+LT thickness) is in the range of not less than 5% and not morethan 40%, variations in electromechanical coupling coefficient k₂ aresmall. The electromechanical coupling coefficient k₂ is proportional tothe band width of a filter. Therefore, because variations inelectromechanical coupling coefficient k₂ are small, if the electrodethickness/(electrode thickness+LT thickness) is within the range of notless than 5% and not more than 40%, variations in band width can beeffectively reduced. Therefore, it is possible to provide apiezoelectric bulk wave device with relatively small variations infrequency.

The thickness of a piezoelectric thin plate is proportional to theacceleration voltage of an ion beam. Generally, an ion implantationdevice has a tradeoff relationship between the beam current andacceleration voltage of an ion beam. This is due to the followingreason. That is, as the electric power value expressed as the product ofbeam current and acceleration voltage is increased, the amount of energyapplied to the substrate per unit time increases, leading to problemssuch as breakage of the substrate due to thermal stress caused by localheat generation. When the acceleration voltage is increased, the depthof ion implantation can be increased, and thus the piezoelectric thinplate can be made thicker. However, because the beam current becomessmaller, it takes longer to implant ions to a concentration that allowsseparation of the piezoelectric thin plate (for example, 8×10¹⁶ions/cm²). Therefore, in order to increase production efficiency perunit time, it is necessary to increase beam current, and it is desirableto decrease acceleration voltage. In order to decrease accelerationvoltage, it is necessary to reduce the thickness of the piezoelectricthin plate to obtain required frequency characteristics. Although theresonant frequency of a piezoelectric bulk wave device tends to becomehigher as the piezoelectric thin plate becomes thinner, the presentinvention proves advantageous in this regard because a piezoelectricbulk wave device for use in a frequency band lower than 1.5 GHz, inparticular, can be realized by means of a piezoelectric thin plate witha small thickness of about 1 μm.

In a case where the electrode thickness/(electrode thickness+LTthickness) is higher than or equal to 5%, the absolute value of theelectromechanical coupling coefficient k² can be also made higher than10%. Therefore, it is appreciated that a filter that covers a widefrequency band can be provided.

Incidentally, bulk wave resonators according to related art use AlN as apiezoelectric body. While the relative dielectric constant of AlN isabout 12, the relative dielectric constant of LiTaO₃ is 40.9 to 42.5,and thus about 3.4 times greater. Moreover, the acoustic velocity of thetransversal wave of a bulk wave is low, at 0.68 times in the case ofLiTaO₃ in comparison to AlN. Therefore, in the case of the samefrequency, the same ratio of the electrode thickness/(electrodethickness+LT thickness), and the same impedance, the area of a bulk waveresonator using LiTaO₃ can be reduced to about one-fifth of the area ofa bulk wave resonator using AlN. Therefore, miniaturization of thepiezoelectric bulk wave device can be achieved.

FIG. 5 illustrates the relationship between upper electrodethickness=lower electrode thickness, and the thickness of LiTaO₃, in acase where the above-mentioned laminated structure is configured so thatthe resonator is set to a frequency of 880 MHz. FIG. 6 illustrates therelationship between upper electrode thickness=lower electrodethickness, and damping capacitance C₀. FIG. 7 illustrates therelationship between upper electrode thickness=lower electrodethickness, and electromechanical coupling coefficient k². It is to benoted that the upper electrode thickness=lower electrode thicknessindicates that the first electrode 6 and the second electrode 7 areequal in thickness, and FIGS. 5 to 7 each illustrate the relationshipbetween the thickness of one of these electrodes, and the thickness ofLiTaO₃, damping capacitance C₀, or electromechanical couplingcoefficient k².

FIG. 8 illustrates the relationship between upper electrodethickness=lower electrode thickness and the thickness of LiTaO₃ in acase where the resonator is set to 1960 MHz. FIG. 9 illustrates therelationship between upper electrode thickness=lower electrodethickness, and damping capacitance C₀. FIG. 10 illustrates therelationship between upper electrode thickness=lower electrodethickness, and electromechanical coupling coefficient k².

As can be appreciated from a comparison between FIGS. 5 to 7 and FIGS. 8to 10, irrespective of whether the resonator is set to 880 MHz or 1960MHz, the upper electrode thickness=lower electrode thickness, and thethickness of LiTaO₃, the magnitude of damping coefficient C₀, and theelectromechanical coupling coefficient k² tend to change in a similarmanner.

While the Euler Angles of LiTaO₃ were set as (0°, 73°, 0°) in FIGS. 2 to10, the same effect was obtained also in the case of Euler Angles of(90°, 90°, 0°). That is, it is appreciated that the same effect isobtained also in the case of a crystal orientation in which the c-axisof LiTaO₃ is in close proximity to the plane direction. As has beendescribed above, this embodiment can provide a resonator or filter thatexhibits relatively small fluctuations in band width and also covers awide frequency band. This is due to the fact that because the specificacoustic impedance of W is high in comparison to the specific acousticimpedance of a transversal wave that propagates in LiTaO₃, thereflection coefficient as seen from LiTaO₃ becomes positive. It is to benoted that as illustrated in FIG. 11, the specific acoustic impedance ofa transversal wave that propagates in LiTaO₃ does not vary very muchwith a change in θ of the Euler Angles, and ranges between 24.8 kg·s/m²to 29.7 kg·s/m². The specific acoustic impedance for W is 50.5 kg·s/m².

While the first and second electrodes 6 and 7 are formed of W in theabove-mentioned embodiment, the metal used is not limited to W, but ametal such as Mo, Pt, or Ta with a high specific acoustic impedance incomparison to the specific acoustic impedance of a transversal wave thatpropagates in LiTaO₃ may be used. Alternatively, an alloy mainlyincluding the metal may be used.

The first and second electrodes 6 and 7 may be each formed by alaminated metal film including a plurality of metal films laminated.

The method of forming the first and second electrodes 6 and 7 is notparticularly limited. A suitable method such as electron-beamevaporation, chemical vapor deposition, sputtering, or CVD may be used.

FIG. 1( a) corresponds to the cross-section of the portion taken alongthe line A-A in FIG. 1( b). The piezoelectric thin plate 5 has slits 5 aand 5 b on opposite sides of a direction along the line A-A of therecess 3 a. Accordingly, as illustrated in FIG. 1( a), the piezoelectricthin plate vibrating part 4 is made afloat above the recess 3 a. On theouter side portion of the slit 5 a, a line electrode 8 is formed on theinsulating layer 3. The line electrode 8 is connected to the secondelectrode 7 in a portion not illustrated in the drawing. In addition, aline electrode 9 is formed on the outer side portion of the slit 5 b.The line electrode 9 is formed on the piezoelectric thin plate 5, and iselectrically connected to the first electrode 6 in a portion notillustrated in the drawing. The line electrodes 8 and 9 are each made ofa suitable conductive material. As such a conductive material, Cu, Al,an alloy of these materials, or the like may be used.

A gold bump 10 is provided on the line electrode 9. In addition, avia-hole electrode 11 is provided in the piezoelectric thin plate 5 soas to be electrically connected to the line electrode 8. A gold bump 12is joined to the upper end of the via-hole electrode 11. Accordingly,the piezoelectric thin plate vibrating part 4 can be vibrated byapplication of an alternating electric field from each of the gold bumps10 and 12. In addition, because the line electrode 8 that transmits aprincipal electric signal is spaced apart from the joint interfacebetween the insulating layer 3 and the support substrate 2, the lineelectrode 8 is able to transmit the principal electric signal with lowloss, without being subject to the influence of semiconductor-likeresistance degradation due to diffusion or non-uniformity at the jointinterface or the skin effect.

In the piezoelectric bulk wave device 1 according to this embodiment,the thickness shear vibration mode of the piezoelectric thin plate 5made of LiTaO₃ is utilized for the piezoelectric thin plate vibratingpart 4. Preferably, of the Euler Angles (φ, θ, φ) of LiTaO₃, φ is 0°,and θ is in the range of not less than 54° and not more than 107°. As aresult, good resonance characteristics utilizing thickness shearvibration mode can be obtained. This will be described in more detailbelow.

By the finite element method, a bulk wave oscillator that utilizesthickness shear vibration mode and thickness longitudinal vibration modeusing LiTaO₃ was analyzed. The thickness of LiTaO₃ was set to 1000 nm. Astructure in which electrodes having a thickness of 100 nm and made ofAl are formed on the top and the bottom of this LiTaO₃ was used as amodel. The area over which the upper and lower electrodes overlap wasset to 2000 μm².

Of the Euler Angles (0°, θ, 0°) of LiTaO₃, θ was varied, and the statesof the thickness shear vibration mode and thickness longitudinalvibration mode were analyzed. The results are illustrated in FIGS. 12 to18.

FIG. 12 illustrates the relationship between θ of the Euler Angles, andthe resonant frequency Fr and anti-resonant frequency Fa of thethickness shear vibration mode. In FIG. 12, the solid line indicatesresonant frequency, and the broken line indicates anti-resonantfrequency. FIG. 13 illustrates the relationship between θ of the EulerAngles, and the electromechanical coupling coefficient k² of thethickness shear vibration mode. As is apparent from FIG. 13, theelectromechanical coupling coefficient k² of the thickness shearvibration mode used takes the maximum value of 14.3% when θ is in thevicinity of 80°. In this regard, when θ is in the range of not less than54° and not more than 107°, the resulting electromechanical couplingcoefficient k² exceeds 5%, thus providing an electromechanical couplingcoefficient required for producing the band width of a filter forportable terminal applications. Further, when θ is in the range of notless than 63° and not more than 97°, the resulting electromechanicalcoupling coefficient k² still keeps a large value of 9.5% or more, whichis two-thirds of the above-mentioned maximum value. Theelectromechanical coupling coefficient k² is proportional to the bandwidth of a filter. Accordingly, it is appreciated that by setting θ ofthe Euler Angles to be not less than 54° and not more than 107°, theelectromechanical coupling coefficient k² can be increased, and a bulkwave filter that covers a wide frequency band can be provided.

Therefore, it is appreciated that according to this embodiment, a bulkwave device that covers a wide frequency band can be provided by settingθ of the Euler Angles to be not less than 54° and not more than 107°.That said, depending on the intended application, it is not alwaysdesirable to make the band width as large as possible. However, the bandwidth can be narrowed by adding an electrostatic capacity in parallel toor in series with a bulk wave resonator. Accordingly, in a case wherethe electromechanical coupling coefficient k² is large, the freedom ofdesign can be increased. Therefore, because θ of the Euler Angles is notless than 54° and not more than 107°, and the electromechanical couplingcoefficient k² is large, bulk wave devices for various band widths canbe easily provided.

As is apparent from FIG. 15, the electromechanical coupling coefficientk² of the thickness longitudinal vibration mode that is spurious becomessubstantially zero when θ is 73°. This also agrees with the resultsillustrated in FIG. 17. That is, FIGS. 16 to 18 explain variation of thethickness longitudinal vibration mode that is spurious due to θ of theEuler Angles. FIG. 16 illustrates the relationship between θ of theEuler Angles, and the resonant frequency Fr and anti-resonant frequencyFa of thickness longitudinal vibration. In FIG. 16, the solid lineindicates the results on resonant frequency, and the broken lineindicates the results on anti-resonant frequency. FIG. 17 illustratesthe relationship between θ of the Euler Angles and the electromechanicalcoupling coefficient k² of thickness longitudinal vibration. FIG. 18illustrates the relationship between θ of the Euler Angles andtemperature coefficient of frequency TCF. As is also apparent from FIGS.16 to 18, when θ of the Euler Angles is 73°, the electromechanicalcoupling coefficient k² of the thickness longitudinal vibration modethat is spurious becomes substantially zero, and when θ is in the rangeof not less than 55° and not more than 85°, as is apparent from FIG. 15,the electromechanical coupling coefficient k² of the thicknesslongitudinal vibration mode that is spurious has a very small value of1.5% or less.

Therefore, more preferably, it is desirable to set θ of the Euler Anglesto be in the range of not less than 55° and not more than 85°. As aresult, the response of the thickness longitudinal vibration mode thatis spurious can be made small. Therefore, in a case where apiezoelectric bulk wave device is configured in accordance with theabove-mentioned embodiment, the attenuation characteristics in the stopband of the filter can be improved.

As is apparent from FIG. 14, the temperature coefficient of frequencyTCF of the thickness shear vibration mode is small at 21.4 ppm/° C. whenθ=75°. Further, when θ is in the range of not less than 62° and not morethan 87°, the TCF is small at 30 ppm/° C. or less. Therefore, furtherpreferably, it is desirable to set θ to be within the range of not lessthan 62° and not more than 87°. As a result, the passband or stopband ofthe filter in the piezoelectric bulk wave device 1 is unlikely to shiftwith a change in environmental temperature. That is, it is possible toprovide the piezoelectric bulk wave device 1 that is stable againstfluctuations in frequency.

FIG. 19 illustrates the relationship between the Euler Angle θ andtemperature coefficient of frequency TCF in the thickness shearvibration mode in a case where LiNbO₃ is used. For LiNbO₃, as in thecase of LiTaO₃, the TCF in the vicinity of θ=73° at which theelectrochemical coupling coefficient k² of the thickness longitudinalvibration mode that is spurious becomes small is checked, and the resultindicates that the TCF is 96 ppm/C.°, which is nearly five times aslarge as the TCF of 21.4 ppm/C.° in the case of LiTaO₃. Therefore, itcan be said that it is preferable to use LiTaO₃ as a piezoelectric thinplate.

In FIGS. 12 to 18, a structure in which electrodes made of Al are formedon the upper and lower surfaces of LiTaO₃ was examined as a model.However, it has been confirmed that even if the electrode material ischanged to another metal, although the absolute value of theelectromechanical coupling coefficient k² may change, setting θ of theEuler Angles within the same range as mentioned above can increase theelectromechanical coupling coefficient k², and further, setting θ withinthe above-mentioned desirable range can reduce spurious, or can make theabsolute value of TCF smaller.

Next, an example of a method of manufacturing the piezoelectric bulkwave device 1 will be described with reference to FIGS. 20( a) to 23(c).

As illustrated in FIG. 20( a), a piezoelectric substrate 5A made ofLiTaO₃ is prepared. Hydrogen ions are implanted as indicated by arrowsfrom one side of the piezoelectric substrate 5A. Ions to be implantedare not limited to hydrogen but helium or the like may be used. Althoughthe energy at which ions are implanted is not particularly limited, thisembodiment adopts the following conditions: a dose of 8×10¹⁷ atoms/cm at150 KeV. The above-mentioned ion implantation conditions may be selectedin accordance with the target thickness of the piezoelectric thin plate.That is, the position of the high concentration ion-implanted portioncan be controlled by selecting the above-mentioned ion implantationconditions.

Upon implanting ions, an ion concentration distribution occurs in thethickness direction within the piezoelectric substrate 5A. The portionof the highest ion concentration is indicated by a broken line in FIG.20( a). In a high concentration ion-implanted portion 5 x representingthe portion of the highest ion concentration indicated by the brokenline, the piezoelectric substrate 5A can be easily separated owing tostress when heated. This method of separation using the highconcentration ion-implanted portion 5 x is disclosed in JapaneseUnexamined Patent Application Publication (Translation of PCTApplication) No. 2002-534886.

Next, as illustrated in FIG. 20( b), a temporary joining layer 21 isformed on the side of the piezoelectric substrate 5A from which ionshave been implanted. The temporary joining layer 21 is provided for thepurpose of joining a temporary support member 22 described later, andalso protecting the piezoelectric thin plate 5 that is finally obtained.The temporary joining layer 21 is made of a material that is removed byan etching step described later. That is, the temporary joining layer 21is made of a suitable material that is removed by etching, and does notdamage the piezoelectric thin plate 5 at the time of etching. As such amaterial for forming the temporary joining layer 21, a suitable materialsuch as an inorganic material or an organic material may be used. As theinorganic material, an insulating inorganic material such as ZnO, SiO₂,or AlN, or a metal material such as Cu, Al, or Ti may be given as anexample, and as the organic material, a material such as polyimide maybe given as an example. The temporary joining layer 21 may be a laminateof a plurality of films made of such a material. In this embodiment, thetemporary joining layer 21 is made of SiO₂.

The temporary support member 22 is laminated and bonded onto thetemporary joining layer 21 as illustrated in FIG. 20( c). The temporarysupport member 22 may be made of a suitable rigid material. As such amaterial, a suitable material such as insulating ceramic orpiezoelectric ceramic may be used. In this regard, the temporary supportmember is made of such a material that hardly any thermal stress isexerted at the interface with the piezoelectric substrate, or such amaterial that when the support substrate and the piezoelectric substrateare joined, the thermal stress exerted at the interface between thetemporary support member and the piezoelectric substrate is smaller thanthe thermal stress exerted at the interface between the supportsubstrate and the piezoelectric substrate. Consequently, in comparisonto related art, it is possible to reduce the possibility of a defectoccurring in the piezoelectric thin plate due to thermal stress exertedwhen separating the piezoelectric thin plate. Further, because thesupport substrate is formed on the piezoelectric thin plate afterheating is performed to split the piezoelectric thin plate, as thematerial for forming the support substrate, a material with anycoefficient of linear expansion may be used without considering thethermal stress that is exerted at the interface between the supportsubstrate and the piezoelectric thin plate.

For this reason, the selectively of the combination of the material forforming the piezoelectric thin plate and the material for forming thesupport substrate can be increased. For example, for devices used forfilter applications, it is possible to improve the temperature-frequencycharacteristics of the filter by making the coefficient of linearexpansion of the material of the support substrate significantly lowerthan the coefficient of linear expansion of the piezoelectric thinplate. In addition, by selecting a material with high thermalconductivity for the support substrate, it is possible to improve heatradiation property and electric power handling capability. Moreover, byselecting an inexpensive material, it is possible to reduce themanufacturing cost of the resulting device.

Next, heating is applied to facilitate splitting of the piezoelectricsubstrate 5A at the high concentration ion-implanted portion 5 x. As forthe heating temperature for facilitating splitting of the piezoelectricsubstrate 5A at the high concentration ion-implanted portion 5 x, inthis embodiment, heating is performed by keeping the temperature atabout 250° C. to 400° C.

An external force is applied in that state to split the piezoelectricsubstrate 5A. That is, at the high concentration ion-implanted portion 5x, the piezoelectric thin plate 5 and the remaining piezoelectricsubstrate portion are separated so as to leave the piezoelectric thinplate 5 illustrated in FIG. 20( d), and then the remaining piezoelectricsubstrate portion is removed.

After splitting the piezoelectric substrate 5A by heating, it isdesirable to apply heating treatment for recovering piezoelectricity asappropriate. As such heating treatment, heating may be maintained forabout three hours at a temperature of 400° C. to 500° C.

The heating temperature required for recovering the piezoelectricitymentioned above may be set higher than the above-mentioned heatingtemperature during the splitting mentioned above. As a result,piezoelectricity can be effectively recovered.

Next, as illustrated in FIG. 21( a), an electrode structure includingthe second electrode 7 and the line electrode 8 is formed on thepiezoelectric thin plate 5 by photolithography. Further, a protectivefilm 8 a is formed so as to cover the line electrode 8.

Next, as illustrated in FIG. 21( b), a dummy layer 23 is formed so as tocover the second electrode 7. The dummy layer 23 is made of a materialthat can be removed by etching. As such a material for forming the dummylayer, an insulating film of SiO₂, ZnO, phospho-silicate glass (PSG), orthe like, various kinds of organic films, a metal with high selectivityof dissolution in the lower electrode or a passivation film covering thelower electrode, or the like may be used. In this embodiment, Cu isused. As an etchant used for the above-mentioned etching, a suitablematerial that allows only the dummy layer 23 to be removed by etchingwithout etching the second electrode 7 may be used.

As illustrated in FIG. 21( c), the insulating layer 3 is formed on theentire surface so as to cover the dummy layer 23, the protective film 8a, and the like. The insulating layer 3 may be made of a suitableinsulating ceramic such as SiO₂, SiN, Ta₂O₅, or AlN. Alternatively, aninsulating material such as glass or resin may be used.

Thereafter, as illustrated in FIG. 22( a), the insulating layer 3 isabraded to flatten its upper surface.

Next, as illustrated in FIG. 22( b), the support substrate 2 islaminated on top of the insulating layer 3 that has been flattened.

Next, the temporary joining layer 21 mentioned above is removed byetching, and separated from the piezoelectric thin plate 5.Consequently, the piezoelectric thin plate 5 can be detached from thetemporary support member 22. In this way, as illustrated in FIG. 22( c),a structure is obtained in which the dummy layer 23, the secondelectrode 7, and the piezoelectric thin plate 5 are laminated over thelower surface of the support substrate 2 via the insulating layer 3.Next, as illustrated in FIG. 23( a), this structure is turned upsidedown, and the first electrode 6 and the line electrode 9 are formed onthe piezoelectric thin plate 5 by photolithography.

Thereafter, the slits 5 a and 5 b, and a via-hole-forming electrode holeare formed in the piezoelectric thin plate 5 by etching. Next, asillustrated in FIG. 23( b), the via-hole electrode 11 is formed.

Thereafter, the dummy layer 23 is removed by etching. In this way, thestate illustrated in FIG. 23( c) is obtained, in which the recess 3 a isformed, and the piezoelectric thin plate vibrating part 4 is madeafloat. Lastly, as illustrated in FIG. 1( a), the bumps 12 and 10 areformed on the via-hole electrode 11 and the line electrode 9,respectively. In this way, the piezoelectric bulk wave device 1according to the embodiment mentioned above can be provided. Accordingto the above-mentioned manufacturing method, ions are implanted into thepiezoelectric substrate 5A having a large thickness in advance.Consequently, the piezoelectric substrate 5A can be easily split at thehigh concentration ion-implanted portion 5 x to thereby obtain thepiezoelectric thin plate 5. According to this method, the piezoelectricthin plate 5 with a relatively small thickness can be obtained with highaccuracy.

While the piezoelectric bulk wave device according to the presentinvention can be manufactured by the manufacturing method according tothe above-mentioned embodiment, the piezoelectric bulk wave device maybe manufactured by other methods.

For example, in the above-mentioned embodiment, the piezoelectric thinplate and the remaining piezoelectric substrate portion are separatedfrom each other after bonding the temporary support member 22 onto oneside of the piezoelectric substrate. However, the step of preparing thepiezoelectric thin plate may be performed as follows. That is, thepiezoelectric thin plate may be prepared by performing the steps ofimplanting ions from one side of the piezoelectric substrate made ofLiTaO₃ to form the high concentration ion-implanted potion mentionedabove, joining the support substrate to the one side of thepiezoelectric substrate, and then separating the piezoelectric substrateat the high concentration ion-implanted portion while heating thepiezoelectric substrate, into a piezoelectric thin plate that extendsfrom the one side of the piezoelectric substrate to the highconcentration ion-implanted portion, and the remaining substrateportion. More specifically, the piezoelectric substrate 5A having thehigh concentration ion-implanted portion 5 x as illustrated in FIG. 20(a) is prepared by ion implantation. Next, the first electrode is formedon the side of the piezoelectric substrate 5A from which ions have beenimplanted. Thereafter, the support substrate 2 is joined to the side ofthe piezoelectric substrate 5A from which ions have been implanted, thatis, the side of the piezoelectric substrate 5A on which the firstelectrode is formed. In that state, while heating the piezoelectricsubstrate 5A, the piezoelectric substrate 5A is separated into thepiezoelectric thin plate and the remaining piezoelectric substrateportion in the same manner as in the embodiment mentioned above. Next,the second electrode may be formed on the side of the piezoelectric thinplate opposite to the side on which the first electrode is formed.

Other than by implanting ions into the piezoelectric substrate made ofLiTaO₃ and splitting the piezoelectric substrate, the formation of thepiezoelectric thin plate may be achieved by abrasion of thepiezoelectric substrate, etching of the piezoelectric substrate, or thelike.

The above-mentioned piezoelectric bulk wave device 1 is merely anexample of piezoelectric bulk wave device according to the presentinvention. The characteristic feature of the present invention residesin that the first and second electrodes 6 and 7 of the piezoelectricbulk wave device are each formed by a conductor with a specific acousticimpedance higher than the specific acoustic impedance of a transversalwave that propagates in LiTaO₃, and further, resonance characteristicsaccording to the thickness shear vibration mode are effectivelyutilized. Therefore, the material, shape, and the like of the first andsecond electrodes are not particularly limited. Moreover, thepiezoelectric bulk wave device may be configured so as to have anelectrode structure that functions not only as a resonator but also asvarious band-pass filters.

REFERENCE SIGNS LIST

-   -   1 piezoelectric bulk wave device    -   2 support substrate    -   3 insulating layer    -   3 a recess    -   4 piezoelectric thin plate vibrating part    -   5 piezoelectric thin plate    -   5A piezoelectric substrate    -   5 a, 5 b slit    -   5 x high concentration ion-implanted portion    -   6 first electrode    -   7 second electrode    -   8 line electrode    -   8 a protective film    -   9 line electrode    -   10 bump    -   11 via-hole electrode    -   12 bump    -   21 temporary joining layer    -   22 temporary support member    -   23 dummy layer

1. A piezoelectric bulk wave device comprising: a piezoelectric platethat is made of LiTaO₃; and first and second electrodes provided incontact with the piezoelectric plate, wherein the piezoelectric bulkwave device utilizes a thickness shear mode of the piezoelectric thinplate made of LiTaO₃, wherein the first and second electrodes eachcomprise a conductor having a specific acoustic impedance higher than aspecific acoustic impedance of a transversal wave that propagates in theLiTaO₃, and wherein, when a sum of film thicknesses of the first andsecond electrodes is defined as an electrode thickness, and a thicknessof the piezoelectric plate made of LiTaO₃ is defined as an LT thickness,the electrode thickness/(electrode thickness+LT thickness) is not lessthan 5% and not more than 40%.
 2. The piezoelectric bulk wave deviceaccording to claim 1, wherein each of the first and second electrodes isat least one metal selected from the group consisting of W, Mo, Pt, andTa or an alloy mainly including the at least one metal, or a laminateincluding the at least one metal and that accounts for more than half ofthe laminate in weight ratio.
 3. The piezoelectric bulk wave deviceaccording to claim 1, wherein of Euler Angles (φ, θ, φ) of the LiTaO₃, φis 0°, and θ is in a range of not less than 54° and not more than 107°.4. The piezoelectric bulk wave device according to claim 3, wherein theθ of the Euler Angles of the LiTaO₃ is in a range of 55° to 85°.
 5. Thepiezoelectric bulk wave device according to claim 3, wherein the θ ofthe Euler Angles of the LiTaO₃ is in a range of 63° to 97°.
 6. Thepiezoelectric bulk wave device according to claim 1, further comprising:a support substrate; and an insulating layer on the support substrate,the insulating layer defining a recess, the piezoelectric plate beingdisposed above the recess.
 7. A method of manufacturing a piezoelectricbulk wave device, the method comprising: preparing a piezoelectric platethat is made of LiTaO₃; forming a first electrode in contact with thepiezoelectric plate, the first electrode being formed by a conductorhaving a specific acoustic impedance higher than a specific acousticimpedance of a transversal wave that propagates in the LiTaO₃; andforming a second electrode in contact with the piezoelectric plate, thesecond electrode being formed by a conductor having a specific acousticimpedance higher than the specific acoustic impedance of the transversalwave that propagates in the LiTaO₃, wherein, when a sum of thicknessesof the first and second electrodes is defined as an electrode thickness,and a thickness of the piezoelectric plate made of LiTaO₃ is defined asan LT thickness, the electrode thickness/(electrode thickness+LTthickness) is not less than 5% and not more than 40%.
 8. The method ofmanufacturing a piezoelectric bulk wave device according to claim 7,wherein the step of preparing the piezoelectric thin plate includes:implanting ions from a first side of a piezoelectric substrate made ofLiTaO₃ to form a high concentration ion-implanted portion on the firstside, the high concentration ion-implanted portion being a portion ofhighest implanted-ion concentration; joining a support substrate to thefirst side of the piezoelectric substrate; and separating thepiezoelectric substrate at the high concentration ion-implanted portion,while heating the piezoelectric substrate, into the piezoelectric platethat extends from the first side of the piezoelectric substrate to thehigh concentration ion-implanted portion, and a remaining piezoelectricsubstrate portion.
 9. The method of manufacturing a piezoelectric bulkwave device according to claim 7, wherein: the step of preparing thepiezoelectric thin plate includes implanting ions from a first side of apiezoelectric substrate made of LiTaO₃ to form a high concentrationion-implanted portion on the first side, the high concentrationion-implanted portion being a portion of highest implanted-ionconcentration, bonding a temporary support member onto the first side ofthe piezoelectric substrate, and separating the piezoelectric substrateat the high concentration ion-implanted portion, while heating thepiezoelectric substrate bonded on the temporary support member, into thepiezoelectric plate that extends from the first side of thepiezoelectric substrate to the high concentration ion-implanted portion,and a remaining piezoelectric substrate portion; and detaching thetemporary support member from the piezoelectric plate.
 10. The method ofmanufacturing a piezoelectric bulk wave device according to claim 9,wherein: prior to detaching the temporary support member from thepiezoelectric plate, the steps of forming a first electrode on thepiezoelectric plate, forming a dummy layer to cover the first electrode,and laminating a support substrate on the dummy layer are performed; andafter detaching the temporary support member from the piezoelectricplate, forming a second electrode on a second side of the piezoelectricplate which is exposed by the detaching of the temporary support member,and causing the dummy layer to disappear.
 11. The method ofmanufacturing a piezoelectric bulk wave device according to claim 7,wherein the piezoelectric plate is prepared having Euler Angles (φ, θ,φ), of which φ is 0°, and θ is in a range of not less than 54° and notmore than 107°.
 12. The method of manufacturing a piezoelectric bulkwave device according to claim 11, wherein the θ of the Euler Angles ofthe LiTaO₃ is in a range of 55° to 85°.
 13. The method of manufacturinga piezoelectric bulk wave device according to claim 11, wherein the θ ofthe Euler Angles of the LiTaO₃ is in a range of 63° to 97°.